The invention concerns a method of monitoring a control circuit which has a number of output stages switching inductive loads, where the current flowing through the inductive loads is regulated by clocked driving of the output stages, and where the current created by the decay of the power stored in the inductive loads during the blocking phases of the output stages is conducted over a current path that is parallel to the load and contains a switch. Circuit arrangements for carrying out this method are also within the scope of the invention.
There are already known automotive control systems, such as anti-skid systems (ABS), drive slip control systems (ASR), driving stability regulators (DSR, ASMS), etc., containing control circuits to which are connected a large number of electrically or electromagnetically activated hydraulic valves. The functions of such control systems are critical for the safety of the motor vehicle, because in an emergency situation or when brakes are applied in a panic, the brake pressure is reduced by an ABS system, for example, to prevent the wheels from locking up, and therefore the brake function is temporarily reduced or canceled. Such systems which are critical for safety and their hydraulic and electrical components, including the control circuits discussed here, must of course be monitored continuously for proper functioning.
Such electrically driven hydraulic valves are inductive loads for the associated control circuit. The control circuits have output stages in the form of driver transistors and similar devices which can be switched on and off thus determine the flow of current through the hydraulic valve. When the driver transistor is blocked, the energy stored in the inductive load, i.e., in the coil of the hydraulic valve, tends to maintain the coil current, and therefore, to limit the cut-off voltage, a path for the cut-off current or free-wheeling current must be created parallel to the coil.
By clocked driving of the driver transistor or the output stages and by utilizing the free-wheeling current, the average current flowing when the valve is switched on can be limited to a preset value, when the cycle duration and cycle interruption are appropriately coordinated with each other. In other words, by clocked driving of the driver transistor, closing of the switch in the current arm parallel to the inductive load during the blocking interval of the driver transistor, and modulating the pulse/pause ratio of the drive cycle, a preset average current can be set. The response thresholds for switching the valve and the subsequent holding current can assume different values; reducing the valve current after valve response or switching leads to considerable energy savings. However, such systems require careful monitoring of adequate current values of the response current as well as the holding current in order to promptly detect, with the required certainty, both errors and "creeping" defects due to changes in reference voltage values, etc.
The object of the invention is to develop a method with which functioning and proper operation, maintenance of various current or threshold values can be monitored with a high degree of reliability in such control circuits.